(1) Field of the Invention
The present invention relates in general to audio distribution system, and more particularly to an arrangement for distribution of audio program material to key telephone system station instruments.
(2) Description of the Prior Art
The use of audio program material in telephone systems is well known as may be witnessed by the existence of U.S. Pat. No. 1,159,855, issued Nov. 9, 1915, to Talbot G. Martin, describing a special service telephone system for providing subscribers with music, news, information, etc. from a central distributing source. Provided in this invention is a loadspeaking telephone receiver which may be bridged across the subscriber's line. At a central station, a special transmitting circuit is bridged across the subscriber's line to provide the line with the above mentioned program material. The subscriber, by merely operating a key or throwing a switch, can connect his loudspeaking receiver directly to the transmitting circuit and thus receive the program material.
More recently, the use of music has appeared in the area of key telephone systems to reassure a central office subscriber that a connection to the key telephone system still exists when such subscriber is placed "on-hold" by a key telephone system user. Such an application is described in U.S. Pat. No. 3,909,553, issued Sept. 30, 1975, to Rickard A. Marshall.
A still more recent application of music in telephone systems is found in the EK-308 Electronic Key Telephone System manufactured by TIE/COMMUNICATIONS, INCORPORATED. Disclosed therein is an arrangement for distributing music to central office subscribers placed in a hold condition, in the usual manner noted above sometimes referred to as "Music-On-Hold". Also included in the provision of music to key telephone station instruments in the "on-hook" state sometimes known as "Background" music. Background music finds use in key telephone systems installed in offices or other locations to block out ambient noise.
Background music is provided in the above system from a music source through inputs connected to an isolation transformer and therefrom to control and signal limiting circuitry which govern signal levels and distribute the signal. Additional transformance for isolation are provided between the music control circuitry and each of the associated key telephone stations. Such a system is costly because it employs transformer isolation for each station line.
Accordingly, it is the object of the present invention to provide an improved arrangement for the provision of background music in key telephone systems, that eliminates the use of costly transformers associated with each telephone line.